1. Field of the Invention
The present invention relates to a display element drive circuit and a display apparatus comprising the display element drive circuit, and more particularly to a display element drive circuit which is provided in each display pixel in a display panel and drives a display element based on a driving current corresponding to a gradation signal and to a display apparatus comprising a display panel in which display pixels each having the display element drive circuit are two-dimensionally arranged.
2. Description of the Related Art
There has been conventionally known a self-luminous type display (a display apparatus) including a display panel in which display pixels are two-dimensionally arranged, the display pixel provided with a light emitting element composed of a current control type display element which operates to emit light with a predetermined luminance gradation in accordance with a current value of a driving current supplied thereto like an organic electroluminescent element (which will be referred to as an “organic EL element” hereinafter), an inorganic electroluminescent element, a light emitting diode (an LED) or the like.
In particular, a self-luminous type display to which an active matrix drive mode is applied has a higher display response speed than that of a liquid crystal display apparatus (an LCD) recently widely utilized in various electronic devices, e.g., a portable information device, a personal computer, a television receiver or the like. Further, the self-luminous type display does not have view field angle dependency, and can achieve an increase in luminance/contrast and in fineness of a display image quality. Furthermore, the self-luminous type display does not require a backlight as different from the liquid crystal display apparatus, and hence the self-luminous type display has very advantageous characteristics that a further reduction in a thickness and a weight or a further decrease in power consumption is possible. Therefore, such a self-luminous type display has been actively studied and developed as a next-generation display.
Such a self-luminous type display is constituted of the display element and a display element drive circuit which is composed of a plurality of switching circuits which drive the display element in accordance with each display pixel constituting a display panel, and various drive methods in the display pixel drive circuit have been proposed.
FIG. 19 is a schematic block diagram showing a primary part of a conventional self-luminous type display. FIG. 20 is an equivalent circuit diagram showing a structural example of a display element drive circuit which is applicable to a display pixel in the conventional self-luminous type display.
As shown in FIG. 19, the conventional self-luminous type display generally includes a display panel 110P in which a plurality of display pixels EMp are arranged in a matrix form in the vicinity of respective intersections of a plurality of scanning lines (selection lines) SLp and data lines (signal lines) DLp respectively arranged in row and column directions. A scanning driver (a scanning drive circuit) 120P is connected with the scanning lines SLp. A data driver (a signal drive circuit) 130P is connected with the data lines DLp, and is configured to generate a gradation signal voltage Vpix corresponding to display data in the data driver 1300 and supplies the generated voltage to each display pixel EMp through each data line DLp.
For example, as shown in FIG. 20, each display pixel EMp is configured to have a display element drive circuit DCp and an optical element (an organic EL element OEL). The display element drive circuit DCp includes: a first thin film transistor (TFT) Tr111 having a gate terminal connected with the scanning line SLp and having a source terminal and a drain terminal respectively connected with the data line DLp and a contact point N111; and a second thin film transistor Tr112 having a gate terminal connected with the contact point N111 and having a source terminal to which a ground potential Vgnd is applied. The optical element is provided which has an anode terminal connected with the drain terminal of the second transistor Tr112 in the display element drive circuit DCp and having a cathode terminal to which a low-power supply voltage Vss whose potential is lower than the ground potential Vgnd is applied.
Here, in FIG. 20, reference symbol CP1 denotes a parasitic capacitance (a retention volume) formed between gate and source electrodes of the second transistor Tr112. Further, the first thin film transistor Tr111 is constituted of an n-channel type field effect transistor, and the second thin film transistor Tr112 is constituted of a-p channel type field effect transistor.
In the display apparatus equipped with the display panel 110P constituted of the display pixels EMp having such a configuration, first, sequentially applying a scanning signal Vsel which is on a selection level (a high level) to the scanning line SLp in each row from the scanning driver 120P turns on the thin film transistor Tr111 of the display pixel EMp (the display element drive circuit DCp) in each row, thereby setting the display pixel EMp to a selection state.
In synchronization with this selection timing, when a gradation signal voltage Vpix corresponding to display data is generated by the data driver 130P and applied to the data line DLp in each column, the gradation signal voltage Vpix is applied to the contact point N111 (that is, the gate terminal of the thin film transistor Tr112) through the first transistor Tr111 of each display pixel EMp (the display element drive circuit DCp). As a result, the second transistor Tr112 is turned on in a conductive state corresponding to the gradation signal voltage Vpix, a predetermined driving current flows to the low-power supply voltage Vss from the ground potential Vgnd through the thin film transistor Tr112 and the organic EL element OEL. Thus, the organic EL element OEL operates to emit light with a luminance gradation corresponding to the display data.
Subsequently, when a scanning signal Vsel which is on a non-selection level (a low level) is applied to the scanning line SLp from the scanning driver 120P, the first transistor Tr111 of the display pixel EMp in each row is turned off, the display pixel EMp is set to a non-selection state, and the data line DLp and the display element drive circuit DCp are electrically disconnected. At this time, the second transistor Tr112 maintains the ON state based on the voltage applied to the gate terminal of the second transistor Tr112 and held in the parasitic capacitance Cp, and a predetermined driving current flows to the organic EL element OEL from the ground potential Vgnd through the first transistor Tr112 like the selection state, thereby maintaining the light emitting operation. This light emitting operation is controlled to continue for, e.g., one frame period until the gradation signal voltage Vpix corresponding to the next display data is applied to (written in) the display pixel EMp in each row.
Such a drive control method is referred to as a voltage specification mode (or a voltage application mode) since a current value of a driving current flowing through the organic EL element OEL is controlled to perform a light emitting operation with a predetermined luminance gradation by adjusting a voltage (the gradation signal voltage Vpix) applied to each display pixel EMp (the gate terminal of the thin film transistor Tr112 of the display element drive circuit DCp).
Meanwhile, in the display pixel EMp equipped with the display element drive circuit DCp adopting such a voltage specification mode, when irregularities or fluctuations (deterioration) are generated in element characteristics (a channel resistance or the like) of the first thin film transistor Tr111 having a selection function or the second thin film transistor Tr112 having a light emission drive function in dependence on an external environment (an ambient temperature or the like), an operating time or the like, a driving current supplied to the display element (the organic EL element OEL) fluctuates, and there is a problem that it is difficult to stably realize desired light emission characteristics (display with a predetermined luminance gradation) for a long time.
Further, when each display pixel is miniaturized in order to achieve high definition, irregularities in operation characteristics (a current between the source and the drain or the like) of both transistors Tr111 and Tr112 constituting the display element drive circuit DCp become large. Therefore, an appropriate gradation control cannot be performed, and irregularities are produced in light emission characteristics of each display pixel, thereby resulting in deterioration of a display image quality.
Thus, as a configuration which solves such problems, there has been known a configuration of a display element drive circuit corresponding to a drive control method called a current application mode (or a current specification mode). Although a concrete structural example of the display element drive circuit of the display pixel corresponding to this current application mode will be explained in detail in conduction with the later-described embodiments according to the present invention, this circuit generally has the following configuration and operation (function).
For example, the element drive circuit corresponding to the current application mode is provided with at least a write control circuit which controls writing a gradation signal current corresponding to display data into the display element drive circuit; and a driving current control circuit which controls a current value of a driving current supplied to the display element and its supply state based on a voltage component corresponding to the written gradation signal current. The element drive circuit is configured to fetch the gradation signal current having a current value corresponding to display data with a timing at which a selection state is set by application of a scanning signal having a selection level to the write control circuit, hold the fetched signal as a voltage component by the driving current control circuit, and supply a driving current having a current value based on the voltage component to the display element in a non-selection state. Consequently the display element is operated to continuously emit light with a predetermined luminance gradation.
In such a configuration, the driving current control circuit has a function of converting a current level of a gradation signal current corresponding to display data supplied to each display pixel into a voltage level and a function of supplying a driving current having a predetermined current value based on the voltage level to the display element. Thus, there is an advantage that the driving current supplied to the display element can be prevented from being fluctuated and deterioration of a display image quality can be suppressed by constituting the driving current control circuit of a single active element (the thin film transistor) even if characteristics of the thin film transistor fluctuate.
However, the display element drive circuit adopting the current application mode has the following problems.
In the display element drive circuit adopting the current specification mode, the operation of writing a gradation signal current corresponding to display data in each display pixel corresponds to charging a capacitance component such as a wiring capacitance which is parasitic on a data line, a retention volume or a parasitic capacitance provided in the display element drive circuit of each display pixel to a predetermined voltage.
Therefore, since such a capacitance component exists, it takes time to some extent until the write operation is completed and a current value of the gradation signal current becomes relatively small at the time of a low gradation in particular. Therefore, an operation is delayed or insufficient writing occurs, whereby the display element cannot perform the light emitting operation with an appropriate luminance gradation corresponding to display data in some cases.
Furthermore, various kinds of capacitance components existing in the display element drive circuit of the display pixel have the following problem. For example, a fluctuation is generated in a control voltage which turns on a switching element (a thin film transistor) constituting the driving current control circuit by capacitance coupling between respective electrodes of a transistor constituting the display element drive circuit or between an electrode and a wiring portion, a fluctuation is further produced in a current value of a driving current supplied to the display element with respect to a specified current value of a gradation signal current, and the display element cannot perform the light emitting operation with an appropriate luminance gradation corresponding to display data. As a result, contrast is lowered to deteriorate a display image quality, for example. An influence of the various capacitance components in a concrete circuit configuration will be explained in detail in conjunction with later-described embodiments according to the present invention.